Hands-on electrochemical impedance spectroscopy Discussion session. Literature:

Transcription

1 Hands-on electrochemical impedance spectroscopy Discussion session Literature:

2 Program for the day General electrochemistry (shjj) Relationship between EIS and electrochemical processes (shjj) EIS techniques, (multi-sine, Laplace, etc) (shjj) Pause Interpretation of EIS signals (Effect of double layer capacitance, Angle of tail, Temperature dependencies, Blocking electrode / non-blocking) (johh) Frokost Equivalent circuits for EIS measurements, most used and why. (johh) Data integrity and verification methods (Kramers-Krönig, etc) (johh) Pause Battery characterization based on EIS parameters. (johh/shjj) Examples, questions from students Topic for next DBS meeting (Martin Søndergaard) Laboratorie tur.

3 Contents Introduction Electrode-Electrolyte Interface - The Double Layer Nernst equation and Volmer-Butler equation Electrolyte Conductivity Thermal activation

4 Electrochemistry is an Interdisciplinary Science Electricity Chemistry A simple electrochemical reaction: Metal electrode Electrolyte containing R R Electrochemistry Materials Science Physics Mathematics e- O R O + e- Electrochemical reactions largely takes places at interfaces chemistry and physics of surfaces & interfaces important!

5 Electrochemistry Some Applications Environmental Corrosion O2(g) Electroplating Water droplet e- OH-(l) Fe2+(l) Cu2+(aq) Direct oxidation of organics Electrokinetic remediation Sensors / Analytical O2(g) + 2H2O(l) + 4e- -> 4OH-(aq) Fe(s) -> + Fe2+(aq) + 2eFe2+(aq) + 2OH-(aq) -> Fe(OH)2(s) 4Fe(OH)2(s) + O2(g) -> 2Fe2O3 H2O(s) + 2H2O(l) Electrochemical Machining Cu(s) Batteries Energy Storage Clark Electrode: O2 + 4 e + 2 H2O 4 OH (amperometric sensor) Anode: x Li+(l) + x e- + C6(s) -> LixC6 (s) Cathode: LixCoO2 -> Li(1-x)CoO2 + x Li+ + x e- Lambda Probe / Zirconia Sensor: Pt / O2(g) / stab. ZrO2 / O2(g) / Pt (potentiometric sensor) Fuel Cells Cu(s) Fe Electrolysers Chemicals Production by Electrolysis Electrowinning/refinement 2H2O(g) + 4e- -> 2O2-(s) + H2 (g) 2O2-(s) -> O2(g) + 4e2H2O(g) -> O2(g) + H2 (g) Gas Production is a big industry Chlor-Alkali, Hydrogen

6 Double layer Definition Electrode Electrolyte

7 Electrode potentials Definition

8 Standard Hydrogen Electrode (wikipedia) 1. platinized platinum electrode 2. hydrogen blow 3. solution of the acid with activity of H+ = 1 mol dm 3 4. hydroseal for prevention of the oxygen interference 5. reservoir through which the second halfelement of the galvanic cell should be attached. The connection can be direct, through a narrow tube to reduce mixing, or through a salt bridge, depending on the other electrode and solution. This creates an ionically conductive path to the working electrode of interest.

9 Double layer Definition Equilibrium potential : Over voltage : Capacitance : Click to edit Master text styles Second level Capacitive current : Third level Fourth level Fifth level

10 Electrode current Transient Steady state

11 Electrode reaction: Example: H2 evolution on a pt-electrode Adsorption rate depends of the number of free sites

12 Electrode reaction in equilibrium -Exhange current

13 Electrode reaction in equilibrium -Nernst equation Assuming all charged species except e- are present in the solution Click to edit Master text styles Second level Third level Fourth level Fifth level

14 Electrode reaction -reaction rate constant relation

15 Electrode reaction -Volmer-Butler equation

16 Electrode reaction -small overvoltage

17 Equivalent circuit The total current is the sum of two currents Therefore, the equivalent circuit for the electrode/electrolyte interface is a parallel connection between a capacitor and a resistor:

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20 Conductivity in Electrolytes Transition from electronic to ionic conductivity in an electrochemical cell E R I H -+ + Liquid Electrolytes Ionic solvation of NaCl - Solid Electrolytes Liquid Electrolytes ClNa + ClCl- H O Na + Na + View down the 110 plane in YSZ Cl- Na + Cl- Oxide ion Vacancy

21 Defects and Kröger Vink notation

22 An Ion in an Electric Field Probability of an ion having the energy u (Bolzmann distribution):

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24 Electrode Kinetics Free Energy Charge Transfer Reactions have an Activation Energy H2 Arrhenius Equation r Gact 2H+ + 2e- k = Ae( Ga ct / RT ) r Go Reaction coordinate (progress)

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27 Solid Oxide Fuel Cell What is the potential φ across the cell in A: OCV (0 A/cm2) B: Fuel cell operation C: Electrolysis

28 POTENTIAL, VOLT Potential Profiles in Solid Oxide Cells v4 + v2=v3 v1 Electrolyte Potential through the electrode supported cell with no current t 0 v2 POSITION v3 v1 v4 ß O-- à - V4 V1 = Emf Emf = - G/(n*F) + Electrolyte H2 + O--D H 2O + 2e - ½O2 + 2e- D O--

29 Measurement of Electrolytic Conductivity & Equivalent Circuits for Electrochemical Cells E R I - MI MIa ElectrolyteII - R- E+ E- R+ + RE C-d + C+d

30 Equivalent Circuits for Batteries LITHIUM ION BATTERIES NEW DEVELOPMENTS, Edited by Ilias Belharouak Electrochemsitry

31 Equivalent Circuits for Batteries LITHIUM ION BATTERIES NEW DEVELOPMENTS, Edited by Ilias Belharouak Electrochemsitry

32 Equivalent Circuits for Batteries LITHIUM ION BATTERIES NEW DEVELOPMENTS, Edited by Ilias Belharouak Electrochemsitry

33 Mass Transport Effects A E Efinal B C C*i t1 t2 t3 t4 Einitial t0 t Mass Transport Effects in Electrochemical Cells 1. Migration (acts on ions, electric field driven) 2. Diffusion (acts on all dissolved not solvent or balance gas - species, concentration gradient driven) 3. Convection (acts on all species, pressure gradient driven / depends on velocity of solution/gas) Diffusion in a linear diffusion field (1-D concentration gradient): Ji = D c x Nernst-Planck (Diffusion, Migration, Convection terms included) J i = D j c j zjf RT D j c j φ + c j v x

34 Polarisation of a Fuel Cell Secant Resistance: ASR = OCV U cell jcell 1.2 Emf= 1 IR-drop E [V] ηcharge-transfer 0.4 ηdiffusion iv curve A j/jlim Ucell / V Current Density / A cm-2 Differential Resistance: ASR = U cell jcell Activation region Ohmic region Mass transport region jcell 1.2

35 Concluding Remarks You are welcome to contact me if you have questions, or want more information, reading tips etc Søren Højgaard Jensen; (office) Further Reading Electrochemical Methods, A. J. Bard & L. R. Faulkner, 2nd ed., Wiley, 2001 Electrochemistry, C. H. Hamann, A. Hamnett, & W. Vielstich, 2nd ed., Wiley-VCH, 1998 Fuel Cell Fundamentals, R. O Hayre, S-W. Cha, W. Colella, F. B. Prinz, 2nd ed., Wiley, 2006